An electronic assembly for detecting the presence of police radar signals is generally known, and will be referred to herein as a radar detector. In use, the radar detector is mounted in a vehicle and provides an audible and/or visual indication of the presence of a police radar signal.
Many known radar detectors cover two signal bands, namely the X band (10.525 GHz+/-25 MHz) and the K band (24.15 GHz+/-50 MHz). Other known radar detectors cover three signal bands, namely the X band, the K band, and a narrow Ka band (34.3 GHz+/-100 MHz).
More recently, the Ka band has been widened and is now specified to be 34.7 GHz+/-500 MHz. However, a problem has arisen is trying to cover this widened Ka band. A radar detector generally has either a fixed or sweeping first local oscillator that emits a signal centered around 11.559 GHz. The third harmonic of this signal (3.times.11.559 GHz=34.677 GHz) falls within the wide Ka band. This signal is radiated out from the antenna of the radar detector and may be received by other radar detectors. If this signal is fixed, it appears to other radar detectors to be a police Ka radar signal and therefore causes these other radar detectors to generate an alert.
As can be appreciated, the only difference between a valid police Ka band radar signal and an interfering signal caused by another radar detector is that the interfering signal has energy radiated at the fundamental frequency and the second harmonic frequency in addition to the third harmonic frequency. Thus, it would be desirable to have a radar detector that could simultaneously determine if there was energy present at the fundamental and second harmonic frequencies when a signal was detected in the wide Ka band, and not alert under these conditions.
Furthermore, in recent years, a number of automatic door openers have been designed to use microwave signals to detect the proximity of people. Although these signals usually appear as X band sources to radar detectors, a group of x band door openers may have the signal properties associated with a k band source. Accordingly, a new false signal rejection scheme is necessary.
A disclosure of the general operation of police radar and police radar warning receivers is provided in U.S. Pat. No. 5,079,553, which is commonly assigned to "Cincinnati Microwave, Inc." (hereinafter referred to as "CMI") and is hereby incorporated by reference. U.S. Pat. No. 5,079,553 discloses a police radar warning receiver including a DSP circuit having a correlator and peak detector. The output of an FM discriminator is digitally sampled so that the magnitude of each digital sample word corresponds to the magnitude of the signals and noise received at the X and/or K band frequencies. Each sample word is then manipulated in a digital correlator and coupled to an averager which performs accumulating and averaging operations for each sample interval or group of intervals. A peak detector compares averager words with a current dynamic threshold. To avoid false alarms, the DSP circuit includes an index memory operating in conjunction with the peak detector to provide sweep-to-sweep comparison. If none of the averager words exceed the dynamic threshold and one or two of the same averager words present the largest magnitude for an extended period of time, an alarm enable is provided. Also, the peak detector evaluates the spacing between those segments which have magnitudes exceeding the dynamic threshold to determine whether the alarm enable should indicate an X or K band.
U.S. Pat. No. 5,068,663 discloses a radar detector which utilizes an amplitude detection scheme to detect radar signals. As shown in FIG. 1 of that patent, the radar detector 100 monitors the X, Ku, K and Ka bands. Amplitude signals are down-converted by a series of mixers and compared to a threshold. Detected amplitude signals must persist for a minimum period of time before the microprocessor 128 performs signal verification.
U.S. Pat. Nos. 4,929,954, 4,772,889, and 4,723,125 disclose devices for calculating a discrete moving window Fourier transform for use in the processing of a pulse compression radar signal. As shown in FIG. 1 of U.S. Pat. No. 4,772,889, a plurality of stages (E) receive samples of the signal x(t) for which a Fourier transformation is sought. To reduce the number of operations performed when the number of stages (E) becomes large, the complex rotation performed by the operator 1 is broken down into a first rotation in the first quadrant that is performed in a way common to all of the stages. Then an additional rotation for each stage equal to 0, 1, 2, or 3 times pi/2 is performed.
U.S. Pat. No. 5,099,194 discloses a digital frequency measurement receiver having an improved bandwidth. As shown in FIG. 1 of the patent, RF signal 10 is mixed with a signal from a local oscillator 12 and then provided to power dividers 32. The mixed signal is divided and supplied to analog to digital converters 42 and 44. Each converter operates at a different sampling frequency. The signal is then supplied to a processor 50 where a Fourier transform is performed to determine a frequency f.
The ESCORT and PASSPORT radar detector products, manufactured by Cincinnati Microwave, Inc., use a correlation scheme to detect the presence of a single period sinusoid, or s-curve. The signal is converted to a digital equivalent with a single bit of precision. Identifiable sets of 0's and 1's will result from the sinusoid or its 180 degree out of phase equivalent. These are conveniently recognized by a low gate count digital circuit. The digitized result is correlated by counting the number of occurrences of 0's followed by 1's. A detection occurs when at least 16 0's are followed by at least 16 1's. The opposite case will also generate a detect and is represented by 16 1's followed by 16 0's.
The NEW ESCORT radar detector, also manufactured by Cincinnati Microwave, Inc., was designed to take advantage of techniques available in spectral processing. It focused on measuring spectral content of portions of the FM demodulator output data collected during the sweep. The detection criterion was chosen to see if the amplitude of the s-curve component exceeded a threshold.
Detecting signals in a wideband creates problems that are not overcome by the prior art. A wideband radar detector picks up the 1000 MHz wide Ka band as well as the X and K Bands handled by more primitive products. The Ka band is 5 to 10 times wider than X and K bands. If the sweep time is held constant, a Ka sweep would then produce an s-curve that is 10 times higher in frequency than that of the X/K sweep. Equivalent analysis processing would require 10 times the throughput. Additional complications arise when the competing considerations of product cost and product sensitivity are taken into account.
Thus, a low cost but high throughput process is needed. Also, for flexibility, the process should be optimized in a software setting.